Environmental issues regarding the use of petrol as an automobile fuel and also the risk that todays oil wells in the future will run dry has led to an intense research regarding an alternative to the use of petrol. Ethanol has been found to be a good alternative to petrol since it to a large extent can be used instead of petrol without major changes of combustion engines. Ethanol can be used today to replace some of the fuel with very small or even without any adjustments at all to the engines.
Strains of the genus Saccharomyces are used widely in the industry for brewing, distilling, baking and various other applications. Saccharomyces cerevisiae is one of the most widely used microorganisms in industrial applications in view of it's ability to convert sugars such as glucose and sucrose to biomass, and fermenting these sugars to ethanol. Strains of Saccharomyces cerevisiae are used in the fuel industry in view of their ability to rather rapidly convert sugars into ethanol and since Saccharomyces cerevisiae has a better tolerance towards fermentation inhibitors and ethanol compared to bacteria and other yeast.
Unlike bacteria and several yeast species, wild-type Saccharomyces cerevisiae is not able to use pentoses such as xylose and arabinose as carbon source. The ability of Saccharomyces cerevisiae to grow on abundant carbon sources such as side streams and waste material from other processes, such as agricultural waste material from e.g. maize and bagasse, and waste material from e.g. paper manufacture, is of great environmental, but also economical, value. Agricultural waste comprises a rather large fraction of hemicellulose, which contains many different sugar monomers. For instance, besides glucose, these sugar monomers can include xylose, mannose, galactose, rhamnose and arabinose. Xylose is the sugar monomer that is present in the largest amount and thus represents an important carbon source for the manufacturing of ethanol using yeasts, providing a huge economic and environmental advantage.
Genes encoding enzymes giving the ability to use xylose as carbon source have previously been introduced in Saccharomyces cerevisiae. EP 1 282 686 discloses recombinant Saccharomyces cerevisiae strains having incorporated genes for the enzymes xylose reductase, xylitol dehydrogenase and xylulokinase as well as having been subjected to a specific mutation. Said strains have the ability to ferment lignocellulose raw materials to ethanol. The strain deposited in Ep 1 282 686 is CBS 102679 (TMB3400, Taurus 01) and is generally recognised to be efficient in the prior art. The ethanol produced by the strain CBS 102679 has been considered very good compared to other prior art recombinant yeasts, but there is also a production of the undesirable byproduct xylitol. Therefore, there is still a need within the art to provide new strains of Saccharomyces cerevisiae having an even better ethanol production, better xylose conversion as well as lower xylitol production in view of the increasing environmental aspects of society today.